My dog has a bump on his neck?

I just noticed it... I don't think it's been there before today. I could be a bug bite? Misquito? I'm not sure. The thing I'm worried about are tumors. He's almost 5 months, he's a yorkie and he weighs 3 pounds. I don't know anything about tumors in dogs, and I researched a bit but it didn't help. It's on the back of his neck, just a round bump. Small, but big enough for me to notice. It's not squishy. He's been scrathing alot.. could that have something to do with it? Thank you